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united.com/preordermeals

Generally available on all flights (including widebodies) over 900 miles except Premium Transcon (EWR<>SFO/LAX)

Pre-order has priority on the flight even about GS/1K and while originally pre-order was limited to a portion of each inflight choice, recent reports pre-order can consume the entire flight's allocation of a particular offering.

archive thread: UA preorder premium cabin meal, Y snacks/drinks {Archive}